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There were two different DAT file formats used for the Fallout games. Both Fallout 1 and Fallout 2 used different formats but used the same file ending: *.dat. To avoid misunderstandings we will refer to DAT1 (for the Fallout 1 DAT format) and DAT2 (for the Fallout 2 version) in this document. It's important that DAT2 is not an improved DAT1 version but more a complete rewrite that does not have much in common with DAT1.

You can see Fallout's DAT2 files as if they were common ZIPs, they're used to store the files that will be used later inside the game, you can store anything you want, compressed or not, having the length you want from a common TXT to an immense MVE, and if you want, you can include a nice but useless DLL file. At the bottom of the DAT is all the included information about each one of these files and some information about the DAT itself.

DAT2 files are divided in 3 parts, Data Block, Directory Tree and Fixed DAT Information block. Data Blocks contains all the files stored in the DAT, some of them needs to be GZipped, others don't. The Directory Tree contains all the information about each file stored in Data Block, as well as the offset where it's located, if it's compressed or not, packed/unpacked sizes, etc. And finally the Fixed DAT Information block that contains the size in bytes of both full DAT and the Directory Tree.
Here you can see a small scheme of how DAT's structure:

The Data Block contains just plain files, their technical information is located in the Directory Tree. Data Block starts from the very beginning of a DAT file. They can be compressed or not, (Fallout engine uses zlib stream data compression), if they're compressed the signature 0x78DA appears at the begin of the file, if not, there is no signature, the file starts without signature. The 0x78DA compression signature has an integer (2 bytes/WORD) nature. 0x78DA in ASCII is "xÚ" as char is 120 for 'x' and 218 for 'Ú' Compressed files are "zlib stream data" (RFC-1950(zlib format), RFC-1951(deflate format), RFC-1952(gzip format)). However, if you attach this header 1F 8B 08 08 9F E8 B7 36 02 03 to the file, such file could been easily decompressed with WinZip.

Directory Tree contains entries that specifies about a file stored in the Data Block. These entries can be varying depending on the FilenameSize of the file (Path + Filename).
Like you saw in the scheme located at the beginning of this document, Directory Tree has been divided into 2 parts, FilesTotal and the DirTree. FilesTotal contains how many files are stored in the DAT, DirTree contains all the information about these files.
FilesTotal is declared as a DWORD (4 bytes/Long) type and is readed in INTEL L-H format.
Format of DirTree entries
DirTree has a private structure. The length of this structure can vary depending on the length of the Filename (path + filename). All the entries are DWord types unless it's specified. At the end of this chapter you can find a scheme on the structure and the way it's declared on C and Visual Basic programming languages.
All the directories and files are stored in DOS 8.3 format, that is 8 characters for the file name and 3 characters for the file extension. All the entries are sorted alphabetically in a descendent direction.
Structure scheme: all Dwords are readed in INTEL L-H format.

Name

Type

Description

FilenameSize

Dword

Length of the ASCII filename.

Filename

String

Path and name of the file, For example, "text\english\game\WORLDMP.MSG".